Electrolytic polishing of metals



Jan. 30g 195i J. MAzlA 2,539,455

ELECTROLYTc PoLIsHING oF METALS Filed Jan. 27, 1944 l 2 Smets-sheet 1 T l'mjPI-i MAziA,'

Jan. 3o, 1951 J. MAZ 2,539,455

ELEcTRoLYTIc PoLsHING oF mnmLs Filed Jan. 27, 1944 2 Sheets-Sheet 2 flllllflllll LTDEEFHM AzlA,

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satented Jan. 30, 11951' UNITED STATES- PATENT OFFICE (Granted under the act f March 3 188.3, as amended April' 3 0, 1928; 370 O., 757)' The invention described. herein may beY manne factured and usedA byor for the Government for governmental purposes Without the payment to me of any royalty thereon.v

The presentA invention relates to electrolytic.

polishing of metals.

By electrolytic polishing of metal is meant the imparting of a lustrous finish or mirror-like surface to metal4 by the conjoint action of a suitable electrolyte and direct electric current passing' from` the metal, undergoing polishing to the electrolyte and then to an4 electrode in contact with the electrolyte. appended claims, a direct electric current, during an electrolysis, is considered as owing from the anode through-the` electrolyte tothe cathode. I n the electrolytic polishing of metal referred to above-there. is, actual smoothing of they metal surface. Light incident upon an electrolytical-ly polished surface experiences regula-r or specular reflection in contrast to irregular Ior diffuse re.

fleet-ion which occurs when light strikes a rough or matsurface.

Heretoiore, electrolytic; polishing has been performed by immersing` or submerging/ the metal` to be polished in a bathk of a suitable electrolyte, and passing'` a direct electric current from the met-a1, as anode, through the electrolyte to a cathode also immersed in the b ath. Agitating or vibrating the metal or Work dur-ingthe electrolysis has been found to be advantageous in avoiding streaks, undulations and coruscations. Various electrolytes for use in electrolytic polishing have been proposed, and indeed the trend of the art of electrolytic polishing has been mainly in the direction of developing of suitable electrolytes for use as the bath. Orthophosphoric acid,y mixtures including orthophosphoric acid, and mix-vv tures of acetic anhydride and perchloric acid have been proposed and used in the past with consider-A able success asl the electrolyte forV the bath.

It is to be appreciated here that r-:lecl'lrolyticV polishing involves something more than a mere electrolysis performed with the work (the metal to be polished) as the anode. f Everyy electrolysis does not necessarily result in the specular polish-kr ing of the metal. Although metal may be dissolved from the work during an electrolysis, thev Ilhe electrolyte must .be one which causes the.

ridges' or high points of' rough, du11,or mat metal surfaces to dissolve away faster during the elec- I- n thisy description, and in tineY The electrolyte.

trolysis than thev metal valleys or depressions between the ridgesor highpoints. Asv the ridgesv Qrhigh. points are dissolved away and. become less in height, the metal surface becomes. more lustrouspolished or mirror-like. Such an elec,-Y

trolyte is. referred to herein asI an electrolytic polishing. electrolyte. order to distinguish -it from electrolytes general-1y, and it may consist of one or more chemical substances.

Electrolytic; polishing is therefore to be care.

fullyV distinguished from electrolytic` etching and electrolytic cleaning of 316i/als. In electrolytic etching of rnetahthefwork ormetal to be etched ismade the anode in any electrolytic cell and met?, al is. Caused, by the. flow of, the, electric Current, to be dissolvedy away from they work without iin-A proving the. specular reflecting .Characteristicsof the resulting. surface. The. resulting surface is a mat one and more, or less resembles the dull surfacey obtainable: by treating a metal surface with an acid capableof dissolving the metal. lillectrolytic` cleaning. is concerned, With removing surface contaminations, such as oxides and grease Or oil, from metals. Although some metal,

as such, mayA be dissolved from the metal during electrolytic cleaning, the, resulting, metallicI surfaces obtainedI in the past have not been iin,-A

proved in lustrousor mirror-like characteristics.`

Amonetheobiets: Qi th present. invent. nis

the provisien of.` a; new and, improved process of, electrolytically poli s ing metals4 Whfh enabl;

the metal or work to: be polished without the use.

of the con-ventionalbath or body of electrolyte refferreol to` above,

Another objectv of the present invention is the provision of a. process which enables the electro: lytic polishingl of discrete, localized, or selec-ted areas ofk large, heavy or cumbersome objects which are not' readily amenable to eithenme cha-nical. polishing on toelec-trolytic polishing involving immersion in a body of an electrolyte containedinr a tank or othersuitablecontainer.

A further object of the present invention is the provision of a process for electrolytic polishing of'` trol of the temperature of the electrolyte is necessary in most instances of polishing.

A further object of the present invention is the provision of a process of electrolytic polishing of metal which can be effectively controlled during use by the mere regulation of an applied voltage.

A further object of the invention is to provide a process for quickly and easily electrolytically polishing metals.

A further object of the present invention is to provide an eiective and rapidly acting process for preparing metallographic specimens with a smooth mirror-like or polished surface, whether as a preparatory step to etching in the determination of grain size, or as a preparatory step for any other purpose.

In the present invention the work or metal is made the anode in a circuit of direct current electricity, but the work is not required to be immersed or submerged in the electrolytic polishing electrolyte as in the tank method of electrolytic polishing, nor is a bath of electrolyte employed within Vthe ordinary meaning of that term. According to the present invention a thin layer of an electrolyte suitable for use in electrolytic polishing is applied in any suitable way to the metal object or area of metal surface to be polished, and a direct current of electricity is passed from the metal object through the thin layer of electrolyte to an electrode in electrical contact with the said thin layer of electrolyte but out of electrical contact with the metal object or area of metal surface. The current path from the anode, the work or metal to be polished, to the cathode is, therefore, very short in the present invention. While the electric current is passing across the thin layer of the electrolyte from `the work as the anode to the cathode, the work and cathode are moved relative to each other, so that during the said relative motion the said thin layer of electrolyte is present between the metal being polished and the cathode.

YAn advantageous and effective manner of applying the thin layer of an electrolytic polishing electrolyte to the metal surface, and at the same time 'establishingin contact therewith a cathode held apart from the work to prevent short circuiting, is to face Yor envelop the cathode with a thin layer of liquid-retentive or liquid absorbent, material, then to impregnate or saturate the said material with the electrolyte and then to bring the metal surface to be polished and the so covered cathode together while a difference of direct current electrical potential exists between the work, as anode, and the covered or enveloped cathode. The thickness or thinness of the liquidretentive material on the cathode automatically determines the thickness of electrolyte through which the current passes in going from the anode to the cathode. The facing or envelope of liquidretentive material is fixed or attached to the cathode Iso'that during the relative motion between the'work or anode and the cathode, the thin layer of electrolytic polishing electrolyte is maintained between the work and cathode. The liquid-retentive, or absorbent material, in the absence of the impregnating or saturating electrolyte, is a\ non-conductor of electricity. Glass cloth, glass wool, asbestos cloth or asbestos wool are suitable materials for use for facing or enveloping the cathode. The thickness of the glass or asbestos cloth or wool applied between the work and the cathode may range from 0.003 to 0.250 inch in practical application of the invention.

The liquid-retentive material on the cathode may be impregnated with the electrolytic polishing electrolyte by dipping the cathode into a body of the electrolyte or by feeding of electrolyte to the solution-retentive material intermittently or continuously in regulated or regulable amounts.

The relative motion between the work, forming the anode, and the cathode, carrying the liquid-retentive material, may be brought about (l) by moving the cathode over the surface of the metal or object to be polished, or (2) by moving the object to be polished over the cathode, or (3) by moving both the object and cathode. The motions may be imparted either manually or by suitable mechanical means. In the second and third methods of applying relative motion, the cathode may consist conveniently of a disk or flat plate of an electrically conducting material such as copper, brass, or steel, over which is stretched one or two layers of glass cloth of a thickness of from 0.003 to 0.005 inch.

In general, inorganic materials of the type of glass cloth or glass wool and asbestos cloth or asbestos Wool are preferred in the practice of the invention, since materials of this type are more resistant to strong acid electrolytes than cloth, wool or felt of animal or vegetable fibres. The use of solution-retentive inorganic materials in the practice of the invention givesl very good polishing results. Wool and cotton woven materials or fabrics also give good polishing eiect, although these materials are more subject to disintegration or decomposition by strong electrolytes than Wools, cloths, or woven fabrics of inorganic material. Organic materials should not be used with electrolytic polishing electrolytes containing perchloric acid, because of the eX- plosion hazard engendered thereby.

The present invention is not conned in its practice to the use of any particular electrolytic polishing electrolyte. The electrolytic polishing electrolyte will be, or should be chosen, according to the metal or alloy to be polished. An electrolyte that may be suitable for electrolytic polishing of one metal may not be suitable for use in electrolytically polishing a different metal.

A suitable electrolyte for electrolytic polishingl of copper, /10 gilding metal, 'l0/30 brass and 60/40 brass according to the invention is composed of by volume of orthophosphoric acid (H3PQ4) of 1.69 specific gravity and 5% by volume of sulphuric acid of 1.84 specic gravity. With this electrolyte and glass wool or glass cloth, as the facing or envelope of the cathode, good polishing effects were obtained with a voltage of from 8 to 12 volts across the work and the enveloped conducting material of the cathode.

For polishing aluminum according to the present invention there may be employed an electrolyte consisting of seven parts by volume of acetic anhydride and two parts by volume of an aqueous solution of perchloric acid of 1.48 specific gravity,V

with a potential diierence of about 15 volts across the work to be polished and the conducting solid portion or metal of the cathode.

In polishing stainless steel of the 18-8 type, an electrolyte consisting of 80% by volume `of 1.69 specific gravity orthophosphoric acid and 20% by volume of 1.84 specific gravity sulphuric acid may be used with a diierence of potential across the work and the solid conductive portion,

of the cathode of about 8 volts.

Plain carbon steel may be polished according to the present invention by utilizing, in its practice, as the electrolytic polishing electrolyte a mixtureconsisting of 80% by volume of acetic anhydrideand .20% by-volumeof an 4aqueous solutionpf perchloric acid of 1.48 s pecicg-ravity. A potentialdifference of about 7 volts acrossthe work .and cathode has given vgood polishing `of plain carbon steel with the aforementioned-electrolyte.

In the preceding four examples of electrolytes usable for electrolytic polishing according -to -the invention, the voltages given apply to operation with thickness 'of glass cloth of vabout 0.003 inch on 4the cathode.

Thecathode which carries the solution-retentive material fmay be -an electrically conductive solid material such as steel, copper, brassnic kel, etc. .The shape and size -of the solution-retentive .material and -cathode may be varied -for aapplication of the process to flat as vwell as Yto .irregular or configured metal surfaces.

.The potential diierence that maybe vapplied across .the work, as anode, and the cathode, .in the practice of the process, vmay range Afrom :5 to 40 volts, although itis to be understood that .the practice of the invention is not limited rto -the use of this specific range of potential difference.

An important feature of the invention resides in the fact that current densities ofahigh order are attainable `in the practice of the process, even .with potential differences as low as 5 to 15 volts across the metal to be polished and the cathode. With potential differences of the order of 5 to 15 volts, current densities of the order of 1000 to 1500 amperes per square foot of anode surface are attainable. This high current density upon fthe locality or area being polished results is very rapid polishing of the metal.

Another important feature of the .present .invention is that the temperature of the electrolyte carried in the solution-retentive material or .applied by it as a thin layer to the metal surface does Anot require regulation or control for successful polishing.

It is vto .be noted that owing to the relative motion between the metal undergoing polishing and the cathode, the thin layer of electrolytic polishing electrolyte upon the metal to be polished is agitated orfsheared during the practice ofthe process. This agitation or shearing breaks up bubbles .of gas liberated during electrolysis, whereby the electrical resistance between anode and cathode is held at a low value, and whereby streaks, vcoruscations and undulations are avoided on the polished surface.

During the vpolishing of metal according to the present invention, metal is dissolved, as explained above, from the high points of the rough or dull surface. The metal so dissolved enters into the thin layer of electrolyte as dissolved salts. Were it not for the agitation or shearing of the thin layer of -electrolyte by the relative motion of the anode and cathode, these salts would tend to form a layer of highly concentrated solution of a salt or of salts immediately adjacent the high as well as Athe low points of the anode,and this layer,.owing to low dissociation of a highly concentrated salt solution, would, without the vagitation or shearing, cause a rise in electrical resistance, and inaddition would deprive the surface of the `metal to be polished of the type of solution necessary for electrolytic polishing. With agitation .or shearing of the thin layer of electrolytic polishing electrolyte, the concentrated salt solution 'is removed preferentially from the high points but retained in the low points. The retention of this concentrated salt solution in the low points retards the dissolution of metal from these points and 'thereby accelerates the .diierentialaction of the electrolytic polishingelectrolyte.

Apparatus suitable for use in practicing the in` vention is illustrated in the accompanying drawings. In these drawings, wherein the same reference numeral indicates the same or .like parts.

Figure l is a plan View of one form of cathode.

Figure 2 is a longitudinal section on the line 2 2 of Figure *1.

Figure 3 is a plan view of another form of cathode.

Figure .tis a section on the line 4--4 of Figure 3.

Figure `5 illustrates the application ofthe invention in electrolytically polishing brass care--` tridge cases of artillery ammunition.

Figure 6 illustrates the application of the invention in electrolytically polishing or lapping a metallographic brass specimen prior to etching in the determination of grain size.

Referring to Figures l and 2, numeral lI indicates a bar of electrically conducting solid ma terial, such as copper, brass or steel. Around one end -of this bar is wrapped a body of glass wool 2, which is secured to the bar'by means of copper wire ties 3. At the opposite end of the bar YI there is provided a binding post or connection 4 for connecting the wire 5 electrically with the bar. The wire 5 may be connected to the negative side of any suitable ysource of direct electrical current.

The cathode illustrated in .Figures l and 2 is adapted to be used manually in polishing by manually kholding the part of the bar between the binding post and glass wool and rubbing the part enveloped with glass ywool over the metal surface to be polished, the glass lwool having" been previously wetted with an electrolytic polishing electrolyte.

The form of cathode illustrated" in Figures 3 and 4 consists essentially of a brass or stainless steel disk I0 and a taut covering consisting of thin cloth II made of woven glass threads. The disk Ill is provided with an annular groove -I12 somewhat inwardly of its edge. A ring I3 flts somewhat loosely in the annular groove I2. The glass `cloth II passes under the bottom of the ring I3 in the groove I2. Four screws I4, -approXimately degrees apart, hold ythe ring I3 tightly down inthe groove i2, by engagement of their heads with the top of the ring I3. The ring I3 serves as clamping Imeans for holding the glass cloth in tautcondition on the disk, vand as a confining wall for holding electrolytic polishing electrolyte on the disk. Numeral I5 indicates a binding post for connecting the electrical conductor I3 to the disk 10. The conductor 16 may be connected to the negative side of any suitable source-of direct electric current.

Figure 5 illustratesa suitable'electric circuitfor using the cathode illustrated in Figures 'l and 2 in polishing a brasscase of artillery ammunition. In this ligure the brass case is indicated yby the numeral 20, and the cathode of Figures 1 and 2 is indicated by the numeral 2l. The positive side of a battery 22, or of any other source of direct electric current, is connected to the brass case 2t by the conductor 23 and spring clamp 24. The negative side of the source 22 of direct electric current is connected through a switch 24, va variable resistance 25, and a conductor 2B a with the cathode 2 I. A voltmeter 2 is connected,

as shown, across the conductors 23 and 26 for cathode is wetted with the electrolytic polishing electrolyte and rubbed over the surface of the brass case while the current is flowing. The rubbing of the surface with the wetted glass wool is continued until the desired degree of polishing is obtained.

'Figure 6 discloses the use of the cathode of Figures 3 and 4 in polishing or lapping a metallographic specimen 30, for example, a brass specimen preparatory for determination of grain size or microstructural elements by etching of the polished surface and microscopic examination of the etched surface. The electrical circuit in this ligure is the same as in Figure 1 andthe parts thereof have been indicated by the same reference numerals. The cathode parts in this gure are indicated by the same reference numerals as in Figures 3 and 4. the positive side of the source of direct electric current 22 is connected by means' of aspring clamp 3l, or by any other suitable means, `with the work 35' (the metallographic specimen) which is moved over the stationary cathode. The disk I of the cathode is connected with the negative side of the source 22 of direct electric current by the conductor 26. An electrolytic polishing electrolyte is poured upon the glass cloth Il of the cathode in suiiicient amount to cover it, and then the specimen 3E is rubbed over the surface of the glass cloth. Back and forth and circular motions in this rubbing may be employed, and the rubbing is continued until the desired degree of polishing is obtained.

I claim:

1. In the art of electrolytic polishing of a metal wherein the metal to be polished is subjected yto the combined action of an electrolytic polishing electrolyte and direct electric current, with the said metal as anode, the process which comprises producing relative motion between the metal surface to be polished and a layer of electrically non-conducting liquid-retentive fabric material wetted with said electrolyte, while the said fabric material and said metal surface are in contact and while passing a direct elec-tric current from i said metal surface through the electrolytic polishing electrolyte wetting the said fabric material;

2. In the art of electrolytic polishing of a metal wherein the metal to be polished is subjected to the combined action of an electrolytic polishing electrolyte and direct electric'current, the process which comprises establishing the said metal as a positive electrode in contact with an electrolytic polishing electrolyte, establishing another conductor in contact with said electrolyte'as a negai.

tive electrode, maintaining between and in contact with said electrodes a liquid-retentive electrically non-conductive means but which nevertheless permits the conduction of electricity between said electrodes by said electrolyte, and moving said electrodes relatively to each other in a direction transverse to their distance apart.

l3. In the art of electrolytic polishing of a metal wherein the metal to be polished is subjected to the combined actie-n of an electrolytic polishing electrolyte, with the said metal as anode, the process which comprises establishing the said metal as a positive electrode in contact with an electrolytic polishing electrolyte, establishing another conductor in contact with said electrolyte as a negative electrode, the distance apart oi said electrodes not exceeding about 0.25 inch, interposing between said electrodes liquid-retentive electrically non-conducting means to hold the In this use of the invention'- 8 l said elec-trodes apart but which nevertheless per-ii mits conduction of electricity between said electrodes through said electrolyte, conducting direct electric current from said positive electrode to said negative electrode through'said electrolyte;` and, during the passage of said current, moving Said electrodes relatively to each other in a direction transverse to the direction of flow of current between said electrodes. j

4. In the art of electrolytic polishing of a metal wherein the metal to be polished is subjected to the combined action of an electrolytic polishing electrolyte and direct electric current, the process which comprises establishing the said metal asa positive electrode in contact with an electrolyticv polishing electrolyte, establishing another conductor in contact with said electrolyte as a negative electrode, spacing said electrodes apart ,by an electrically non-conductive liquid retentive material which permits the conduction of elec-'3 tricity 'between said electrodes by said electrolyte, and moving said electrodes relatively to each` other in a direction transverse to their distance apart. v u

5. In the art of electrolytic polishing of a metal wherein the metal to be polished is subjected to the combined action of an electrolytic polishing electrolyte and direct electric current, with the said metal as anode, the process which comprisesestablishing a layer of electrolytic polishing electrolyte between said metal, as anode, and a cath-'- ode spaced from said metal at a distance of fromv about 0.003 inch to about 0.25 inch, said layer on' one side making electrical contact with saidmetal and on the'opposite side making contact with said cathode; preventing direct electrical Contact between said anode and said cathode by inter" posing between and in contact with opposed surfaces of them a liquid-retentive electricallyA non-f conductive material which permits the conduction of electricity between said anode and cath;

ode by said electrolyte; passing direct electric current through said electrolyte from said metal, as anode, to said cathode and during the passageof-said current moving said me-tal and' cathode relatively to each other in a direction transverse to the'distance between said metaland cathode.

REFERENCES CITEDv The following references are of Y'record in the" le of this patent:

UNITED STATES PATENTS OTHER REFERENCES Metai Progress, Dec. 1939, pages 755, 75651'- A-ug. 1942, pages 2,09 through 212; Jan. 1940 pages Canadian Chemistry & Process Industriesj.,`

July 1940, pages 367, 368, 369.

The Metal Industry, Jan. 29, 1943, pages 69, y

Steel, June 17, 1940, pages 72 and 74.

JOSEPH MAZIA."

Sweden July 29, 19411 

1. IN THE ART OF ELECTROLYTIC POLISHING OF A METAL WHEREIN THE METAL TO BE POLISHED IS SUBJECTED TO THE COMBINED ACTION OF AN ELECTROLYTIC POLISHING ELECTROLYTE AND DIRECT ELECTRIC CURRENT, WITH THE SAID METAL AS ANODE, THE PROCESS WHICH COMPRISES PRODUCING RELATIVE MOTION BETWEEN THE METAL SURFACE TO BE POLISHED AND A LAYER OF ELECTRICALLY 